In general, in a differential pressure type leak test, pressurized gas such as compressed air is introduced to an inner space of a workpiece and a reference space. Then the inner space and the reference space are blocked from each other, each forming a closed system. If there is leak in the workpiece, the leak is detected as a differential pressure. In this way, a pass/fail judgment can be made on the workpiece.
When the pressurized gas is introduced into the inner space of the workpiece, the temperature of the inner space is increased due to adiabatic compression. Then, the heat is released with time, and the temperature is decreased. If there is temperature difference between the workpiece and facilities surrounding the workpiece or ambient air due to heating or cooling of the workpiece, or if there is temperature difference between the pressurized gas and the workpiece, the temperature inside the workpiece changes with time. Such a temperature change also causes a pressure change.
To address this issue, in a method for leak testing disclosed in Japanese Published Patent Application No. 2007-064737 (referred to as “Patent Document 1” hereinafter), temperature change, as well as pressure change, in an inner space of a workpiece is measured, and compensation is made in which a component of the pressure change attributable to the temperature change is subtracted from the pressure change. As a result, accuracy in leak judgment and consequently in pass/fail judgment on workpiece can be increased.
To be more specific, a heat sensitive member having high thermal conductivity, for example, is prepared. A sealed pressure chamber (heat sensitive chamber) is formed inside the heat sensitive member. The heat sensitive member is disposed in the inner space of the workpiece. Pressurized gas is introduced into the pressure chamber of the heat sensitive member. Furthermore, the pressurized gas is introduced into the inner space of the workpiece (space between an inner wall of the inner space of the workpiece and the heat sensitive member, to be more exact). Then the pressure change in the pressure chamber as well as the pressure change in the inner space of the workpiece is measured. The pressure change in the pressure chamber is mainly attributable to temperature change in the inner space of the workpiece. Therefore, based on the measured data on the pressure change in the pressure chamber, compensation can be made to the measured data on the pressure change in the inner space of the workpiece, and a component of the pressure change attributable to the temperature change can be subtracted from the pressure change. Leak judgment is made based on the compensated data.